1. Field of the Invention
The invention relates to a controller for an internal combustion engine, and more particularly to a controller for an internal combustion engine of a spark ignition type.
2. Description of Related Art
Japanese Patent Application Publication No. 2012-021501 (JP 2012-021501 A) discloses a controller for an internal combustion engine equipped with a tumble flow control valve that generates tumble flow inside a cylinder. In such conventional controller, an estimated tumble ratio is calculated on the basis of a detection value of a first air flow meter provided upstream of a throttle valve and a detection value of a second air flow meter provided directly below the tumble control valve (TCV). The opening degree of the TCV is feedback controlled such that the calculated estimated tumble ratio follows the target tumble ratio. The target tumble ratio is set to within an allowed control range for avoiding misfire and combustion instability.
Although the tumble ratio (ratio of the flow velocity of the tumble flow to the engine rotation speed) is controlled to within a fixed range, where the engine rotation speed changes the velocity of the tumble flow also changes. During lean burn operation conducted under the condition of a low fuel concentration in an air-fuel mixture, such as operation at an air-fuel ratio (A/F) higher than the stoichiometric ratio or exhaust gas recirculation (EGR) operation in which air containing a large amount of EGR gas is burned, the gas flow velocity around the sparkplug at the time of ignition is either too high or too low, and stable ignition is difficult to obtain.